Research Goals in the next year of this project are: 1. Develop a stable electrode for creatinine in serum using immobilized creatininase (E.C. 3.5.4.-). The enzyme will be purified of all urease and creatinase activity, and bound to a gas NH3 membrane sensor. 2. Optimize the assay system for cholesterol using immobolized cholesterol esterase and cholesterol oxidase bound together on glass bead supports. 3. Develop an assay procedure for mixtures of nitrate and nitrate ion in mixtures using 2 enzyme systems, nitrate reductase and nitrate reductase from E. coli. 4. To develop systems for the assay of phenylalanine in serum using phenylalanine ammonia lyase immobilized onto an NH3 gas membrane electrode. The undersigned agrees to accept responsibility for the scientific and technical conduct of the project and for provision of required progress reports if a grant is awarded as a result of this application. BIBLIOGRAPHIC REFERENCES: G. G. Guilbault, Enzyme Electrodes, in Immobilized Enzymes, H. Weetall, ed., Marcel Dekker, New York, 1976. G.G. Guilbault and T. Cserfalvi, Ion Selective Electrodes for Phosphate using Enzyme Systems, Anal. Letters 9, 277 (1976).